Switched-capacitor regulators are needed in many portable electronic devices to provide supply voltages for different systems. Possible devices include personal digital assistants, notebook computers and mobile phones.
The importance of switched-capacitor regulators is demonstrated by their capability to provide dc-to-dc conversions without the use of inductors. Less conducted electromagnetic interference and cross couplings are then generated to other systems, since no magnetic components are used. Existing control methods adopted in switched-capacitor regulators require extra power transistors or auxiliary circuits to regulate the output voltage. These increase the chip area of the thereby increasing the manufacturing cost.
The accuracy of the output voltage is critical to switched-capacitor regulators. The accuracy is determined by line and load regulations and the load-transient recovery time of regulators, which in turn is dependent on the implementation of the controller. Existing implementations employ single-stage linear amplifier in the controller, which can affect the accuracy of the regulators.
A number of methods have been employed to maintain the output voltage of a switched-capacitor regulator. Both U.S. Pat. No. 6,445,623 and U.S. Pat. No. 6,411,531 disclose two single-ended switched-capacitor regulators and are illustrated in FIGS. 1 and 2 respectively. Additional circuitry (either a current source or a variable resistor connected between the input supply voltage and switches S1, S2, S3 and S4) is required to maintain the output voltage. However, the additional circuitry is implemented by power transistors; thereby increasing the chip area and manufacturing cost.
Similarly, U.S. Pat. No. 6,411,531 also describes a single-ended switched-capacitor regulator as shown in FIG. 3, which uses an extra current source connected between the power switches and ground for output-voltage regulation. Therefore, the area efficiency of the switched-capacitor regulator is not ideal
FIG. 4 shows a cross-coupled switched-capacitor regulator from U.S. Pat. No. 6,618,296. The cross-coupled design is similar to the configuration in U.S. Pat. No. 6,445,623, used to achieve dual phase operation. Extra power transistors are still required for regulating the output voltage, and this lowers the area efficiency of the regulator.
FIG. 5 shows a cross-coupled switched-capacitor regulator which maintains the output voltage without using extra power transistor, as described in W. Chen, W. H. Ki, P. K. T. Mok and M. Chan, “Switched-capacitor power converters with integrated low-dropout regulators”, IEEE International Symposium on Circuits and Systems, Sydney, Australia, Vol. III, pp. 293-296; 2001. Power transistors M14 and Mr4 are alternately turned off during the capacitor charging phase and regulate the output voltage of the control loop during the capacitor discharging phase. Therefore, M14 and Mr4 function as switching low-dropout regulators (SLDR) and the control scheme is known as an SLDR mechanism. By adopting SLDR control, an analog buffer should be used in order to enable M14 and Mr4 to operate in the saturation region during capacitor discharge phase. However, extra auxiliary voltage doubler are required to generate a high-voltage clock signal swinging up to 2VDD so as to drive extra switches connected to the output of the analog buffer. Extra auxiliary voltage doublers increase both the area and total static current dissipation in the controller. In addition, since the gate-voltage swing of M14 and Mr4 is 2VDD and increases with the input supply voltage, the SLDR control causes switching noise problems in the switched capacitor regulator. Moreover, gain stage and analog buffer in the controller cascaded with power transistors M14 and Mr4 results in a two-stage switchable opamp. Two-stage switchable opamps may not provide sufficient loop gain magnitude, and hence the accuracy of the switched capacitor regulator is not optimized.
There is a need for a control method for a switched-capacitor regulator that is able to efficiently and accurately regulate the supply voltage of electronic devices without requiring additional power transistors or auxiliary circuits.